Radar reflective tow target



Filed Jan. 16, 1956 4 Sheets-Sheet 1 I!!! wi l 2057J #02 42 OQSO/VB. 4041144 066 1 IN VEN TORS Nov. 21, 1961 R. J. HOPPER ET AL RADAR REFLECTIVE TOW TARGET 4 Sheets-Sheet 2 Filed Jan. 16, 1956 8055GT J. A OPP'SQ OQSO/V 5. AOL/M4064 Nov. 21, 1961 R. J. HOPPER ET AL RADAR REFLECTIVE TOW TARGET 4 Sheets-Sheet 3 Filed Jan. 16, 1956 M4 06 IN V EN TORS Nov. 21, 1961 R. J. HOPPER ET AL RADAR REFLECTIVE Tow TARGET 4 Sheets-Sheet 4 Filed Jan. 16, 1956 km W.

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we NW4 WWW w B nited States 3,010,103 RADAR REFLECTIVE TOW TARGET Robert J. Hopper, Pacific Palisades, and Orson B. Lolmaugh, Los Angeles, Calif., assignors to Del Mar Engineering Laboratories, Los Angeles, Calif., a corporation Filed Jan. 16, 1956, Ser. No. 559,372 12 Claims. (Cl. 343-18) This invention relates to an aerial tow target for training pilots in the use of equipment for detecting and tracking aerial targets and the use of various automatic equipment for such purposes as fire control, missile launching control and target interception.

The invention is particularly directed to certain problems that are encountered in the task of providing realistic target experience for fighter pilots, interceptor pilots and various other military personnel partcipiating in organized teamwork for detecting, tracking and destroying aerial targets.

Any actual military aircraft has numerous reflecting surfaces extending in random directions, including numerous contiguous reflecting surfaces in intercepting planes. Consequently a military aircraft at any position relative to a radar receiver provides at least one adequate radar reflection back to the receiver, which reflection is of sufficient strength for detection of the aircraft at a relatively long distance.

This capability of an aircraft for radar reflectivity adequately distributed over a relatively large cone angle makes possible not only early detection of the aircraft at a distance but, also makes possible continued unbroken radar surveillance of the aircraft thereafter. Thus, with the aircraft illuminated by radar transmitted from the ground or from the air, this wide angle of effective reflectivity makes it possible for a missile equipped with a radar receiver-guidance unit to be continuously guided by radar reflections from the aicraft for effective control along a desired curved interception path. A vitally important problem in military training, then, is to provide a practice target that is small enough and light enough to be towed by a tow line several thousand feet long and yet will adequately simulate an actual enemy aircraft with respect to reflection of radar signals.

The invention; meets this problem by providing the 2 may be at or nearly 180 out of phase with each other to result in mutual cancellation. The possibility that such neutral cancellation may completely cut off the desired response of the receiver to the target is adequately minimized by using such diversely spaced corner reflectors as to afford practical assurance that more than two random reflections will be returned to a receiver at any given direction from the target.

An important feature of the invention is the concept of including some relatively large corner reflector units along with the numerous relatively small corner reflector units. The larger reflector units are of a dimension more than five times the Wave length of the radar signals. In the preferred practice of the invention, the dimension of the larger reflector units are at least 6 /2 times the Wave length of the radar signals to result in mono-static reflection. As will be explained, the larger corner reflector units may comprise a forward radial assembly and an aft radial assembly with the smaller corner reflector units arranged in a circumferential assembly between'these two radial assemblies.

This combining of a minor number of relatively large corner reflector units with a major number of relatively small reflector units completes the requirements for effectively simulating an actual aircraft from the illuminating position since the mono-static radar reflections from practice target with a plurality of radar corner reflector units, each of which has three reflecting surfaces that are normal to each other and that meet at a common point at the inner corner of the unit. An exceedingly important factor in the solution of this problem by the invention is that the plurality of corner reflector units includes numerous diversely oriented reflector units that are relatively small. 7 These relatively small reflector units are of adimension substantially less than five times the wave length. of the radar signals employed by the detecting devices.

It has been found that if a'corner reflector unit is of a dimension on the order of three or four times the wave length of the radar signal, the unit produces bistatic" radar reflection, the signal being substantially diffused. The'invention takes' advantage of this fact by providing numerous relatively small radar reflectors. In this manner the tow target of "the invention provides a high probability that a radar receiver located within, say, an 80 cone measured from the direction of illumination (at any given direction from the tow target) will receive a substantial amount of radar reflection from the target.

The invention further takes into consideration the fact that two signals reflected in this manner to a receiver v the larger reflector units may be picked up at relatively great distances for initial detection of the practice target and the bi-static reflections from the smaller reflector units enable a radar-guided missile to remain constantly locked to the practice target along the desired curved intersection path of the missile. The-mono-static reflection patterns from the larger reflector units are desirable for their relatively great amplitude but have null points that are not characteristic of the over-all patterns of radar reflection from actual aircraft. The bi-static reflection patterns from the small corner reflector units fill in these null points to result in a composite reflection pattern that is equivalent in target-guidance effectiveness to the over-all pattern of radar reflection from an act ual aircraft. The diffused bi-static reflections may be r'ela' tively Weak but such reflections are nevertheless adequate for the purpose of the invention since that portion of the interception path Where close guidance control of the missile is necessary is relatively'close to the practice target. 1

The preferred practice of the invention is further characterized by the concept of providing aerodynamic means to cause the tow target to spin on its longitudinal axis. This spinning action increases-the probability that the radar signals will be adequately reflected to a radar receiver in the desired directions. By virtue of the spinning action, only a relatively few of the small corner reflector units may be used to provide substantially uniform reflection of radar signals from the target in the general illuminating direction. a

The preferred practice of the invention is further characterized by the incorporation into the tow target structure of an electronic transmitter receiver for transmitting information pertinent to target practice. For example, such an electronic transmitter-receiver unit may function as a miss-distance indicator to transmit information by radio relative to the trajectory of missiles that miss the tow target. In this regard, a feature of the preferred practice of the invention is the provision of a propellerdriven alternator or generator for ener'gizing the electronic transmitter-receiver, the propeller being actuated by the air stream around the tow target.

The variousfeatures and advantages of the invention suitable cellular will be understood from the following detailed descrip tion considered with the accompanying drawings.

In the drawings, which are to be regarded as merely illustrative:

FIGURE 1 is a side elevation of a typical embodiment of the present invention;

FIGURE 2 is a similar view with portions of the target broken away to reveal concealed structure;

FIGURE 3 is a view of the nose portion of the target, the. view being partly in. side elevation and partly in section;

FIGURE 4 is a transverse section taken as indicated by the line 44. of FIGURE 3 showing how the relatively small corner reflector units are mounted on the inner surface of the target shell to form a circumferential assembly of the units;

FIGURE 5 shows the forward radial assembly of relatjively large corner reflector units, the view being partly in front elevation and partly in section as viewed along the line 5-5 of FIGURE 3;

FIGURE 6 is a perspective view of the disassembled Thus, a station listens in on the. signals-exchanged by the components of the forward radial assembly of relatively large corner reflector units; FIGURE 7 is a transverse sectional view-illustrating a modification of the invention in which a circumferential series of the relatively small corner reflector units are positioned in a random manner zit-various radial distances from the shell ofthe target; FIGURE 8 is a similar transverse sectional view of another modification of the invention in which a circumferential series of the corner reflector units are mounted on 'a ci'rcumferential'ly tapered spacer inside the target manner in which the tow target is constructed to include to the usual tow cable 38 and member is anchore-d'to' a suitable; bulkhead 49 which shell to position the reflector units at various radial'distances inwardly from the target shell; I 7

FIGURE 9 is a perspective view of a corner reflector 'unit modified by a tapered cut for the introduction of a random factorin another modification of the invention; FIGURE 10 is a transverse sectional view illustrating another practice of the invention in which the body of the: target is 'made of foamed cellular plastic, the cellular plastic: being peripherally recessed to receive the circumferential assembly of corner reflector units;

FIGUREll is a graph showing a typical radarreflection pattern asproduced by a corner reflector unit; that is relatively large, in comparison to the wave length or the radar signal;'and

FIGURE 12 is a similar graph showing the bi-static radar reflection pattern produced by a cornerreflector thatis substantially smaller. I

V The presently preferred embodiment of the;tow target illustrated by. FIGURES l to 6 of the'drawiu'gs has a body in the. form ofr" a thin designated by the letter 8.

number of sections comprising a nose section '20, a" tail section 22, and three adjoining intermediatesections 24',

25 and 26. Thecontiguous edges of thesevarioussecscope into the edge of the adjacent section as shown.

streamlined shell, generally 1" I In the construction shown, 1- e the body shell Sis made of paper and molded in a;

section radially inward to tel'a thickened by the addition of a ring 46 of the same mate-j mono-static ,swivel member axialmember. p

36: by two spaced bearings 54' and is atubular shaft-.56

Thus the joints between successive shell sections form in-,

ternal circumferential reinforcement ribs, for example,

as indicated at28 in FIGURE'Z;

Inthe'constructi'on shown, the s'treamlinedbody shell 7 S is rprovided' with a set of four stabilizing airfoils or tail fins '30 which may be of any suitable construction. ,In this instance, e'achofth tail fins '30 is mold'ed' fromfla plastic material such as, expanded stys rene plastic available under the trade name Styrofoam.

v material has a compressive strength of 2880pounds e5 g r 1 nected to a suitablelantenna byacable 65., Thelantepna' may be in the formofla loop and may be. positioned anyper square fo ota'nd y'et Weighss only onefand a hal f to two pounds per cubicioot.

' otth'etow target'shell by suitablefadh'esive plastigf 7 7' Flu this embodiment of the invention, theinose section The molded cellular tail fin Bo -may be slmply 'bonded to the peripheral surface bearingGZ.

, I 50, notonl'ysupports the "statorfof the 'generatorx34 buit I .60"

. V antenna comprises a i ventioni includes relat vely large: radar. corner reflector;

20 of the tow target carries an electronic tnansmitterreceiver in an annular housing 32 to transmit signals, pertinent to target practice procedures and this electronic apparatus is energized by a suitable electromotive source carried by the tow target. In this instance, the electromotive source comprises a suitable generator 34' driven by a forward propeller or air turbine 35 thatis actuated by the surrounding air stream. The transmitter-receiver may be, for example, what is termed a miss-distance in: dicator and for this purpose may send out radio signals a at regular time intervals to be received by transponders on missiles directed at the tow target. The transponders respond by returning a radio signal onv a different frequency to which the receiver part of the transmitter-receiver is tuned. The transmitter-receiver responds to the returned signal immediately by transmitting anew signal l without waiting until the. end of, the regular timeinterval'i tow target and the passing missiles and noting the time intervals of the signals. obtains data: indicative of the dis ture of this particular embodiment of'the invention the thecomponents of thiselectronic apparatus' j i Asbest shown in. FIGURE 3 the structure of thetow t'arg'etincludes an axial tension member 36 for connection 1 therearend or this axial rial, is suitablybondedto thetbodyshell S by plastic adhesive. United with the bulkhead 40 is a series of forwardly extending radial gusset plates 48" and which] maybe integral with the ringgtS. These gus et plates'48 are preferably united with a forward-reinforcement ring j- 50.'-.1 7TH Preferably, the forward end of the axial member'36jis connected to'the tow cable'Bfi by means of a suitable 52 thatis ro'tatably" mounted inside the Suitably journalled on the axial member :that'carries the propeller- 35; a's well as'fthe rotor'pfthe V ner 58 is unitary with the-propeller 35.;

Thefstatjor of the generator 34' s attachedirtdz the gusset plates '48 by radial; webs for vSliPP m :the previously mentioned forward 7 reinforcement I ring 50 journals the V g V Thhs the bulkheadlassernbly; iwhi'ehiinclude's i l i the'g'ussetplates 48 and'thetorward 'reinforcernent ringf also reinforces and holds rigid 'thefffoi wardlend ofthe axial 'me'mber36 by stabiliainggthejorward end-joflthe f V The generator, which incorporates a suitable voltage regulator, is connected to the transmitterareceiver- 32 by" acable 64 and the transmitter-receiver is, in t'urn; con-1 iwhere on the tow target .Intheconstructionfshowm the loopy6'6 that islsandwiched betweenf ,two discs 68 of cellulareplastic, ;thes,eftwo discs beingpo .tionedin ,thejregiomof, the tail fin'seo for re" orce ent' thereofQ-Q ff 'Asiheretoforestate e present in the; targ e I o unitsaswell as relatively small 'radar cforner reflector itiibular'shaft; 56, bynieans ofa suitable ball f a units. In the present construction, the relatively large corner reflector units comprise a forward radial assembly of units, generally indicated by numeral 70, and an aft radial assembly, generally designated by the numeral 72. Since both of these radial assemblies are of the same construc tion a detailed description of the forward assembly will suflice for both.

The structural parts of the forward radial assembly 70 are shown in disassembled state in FIGURE 6. A rela tively thick disc of cellular plastic is cut to form a semicircular piece 74 and two quadrants 75 and 76 and these two quadrants are bonded to opposite faces of the semicircular piece to form a cruciform body generally designated by numeral 78. The purpose of the cruciform body 78 is to house and support a cruciform reflector member, generally designated 80, which may be formed by bonding two sheet metal quadrants 82 and S4 to the opposite faces of a semi-circular piece of sheet metal 85. The sheet metal may -be aluminum or copper, for example. In profile the cruciform reflector member 80 is somewhat larger than semi-circular since the center of curvature of the various arcuate edges is at the point indicated by the arrow 86 and the cruciform reflector extends rearward a short distance from this center.

To permit the cruciform reflectormember 80' to be housed in the cruciform body 78, a circular saw of the same radius of curvature as the reflector member 80 is employed to cut two diametrical kerfs 87 in the cruciform body. It is apparent that the cruciform reflector member 80 will seat in the two kerfs 87 with edge portions of the reflector member protruding rearward from the cruciform body 78. These rearwardly protruding intersecting diametrical edge portions are received by corresponding kerfs 88 in a disc 99 of the same cellular plastic material, the thickness of the disc being selected to place the rearward edges of the cruciform reflector member flush with the rearward face of the disc. A reflector disc 92 of the same metallic material as the cruciform reflector member 80 is then positioned on the rear face of the cellular plastic disc 90 and is'covered by a second cellular plastic disc 94. It will be noted that the reflector disc 92 is of the same diameter as the cruciform reflector member 80 which diameter is less than the diameter of the two cellular plastic discs 90 and 94.

It is apparent that when the plastic disc 90 is bonded to the cruciform plastic body 78 and the second foam plastic disc 94 is peripherally bonded to the plastic disc 90, both the cruciform reflector member 80 and the reflector disc 92 are securely held in place to form a radial array of four relatively large radar corner reflector units, each unit having three planar reflector surfaces that are perpendicular to each other and meet at a common point. It may be noted that the dimension of each of these radar corner reflector units is nearly equal to the radial dimension of the tow target shell S at the particular location of the reflector unit. The corner length of each of these relatively large radar corner reflector units is more than flve times the wave length of the radar signal that is employed in the target runs and, in this instance, is from six and a half to eight times the wave length.

A plurality of relatively small radar corner reflector units 95 are positioned inside the tow target shell S in positions facing outward and the dimension of these smaller radar corner reflector units is substantially less than five times the wave length of the radar signal. In this instance the dimension is three to four timesthe wave length and may be, for example, three and a third times the wave length. Each of the smaller radar corner reflector units 95 may comprise, for example, three cardboard ,triangles joined at their edges to form a hollow triangular pyramid having three inner faces that are mutually'perpendicular and that meet at a common point or vertex, each of these cardboard surfaces being faced or lined with suitable reflecting material such as aluminum 6 foil. These hollow corner reflector units are bonded by their corners to the inner surface of the tow target shell S.

In the present embodiment of the invention, the plurality of relatively small radar corner reflector units form a peripheral assembly between the two radial assemblies 70 and 72 of the relatively larger corner reflector units. Thus as shown in FIGURE 2, the relatively small radar corner reflector units 95 may be arranged in six annular series or circumferential rows inside the tow target shell S.

Since the various relatively small radar corner reflector units 95 are arranged radially, they provide a high probability that a plurality of radar reflections will be directed to a radar receiver that is positioned within a cone centered about the illuminating direction, the transmitter and receiver being located in any given direction from the tow target. Further diversification in the spacing of the plurality of relatively small radar corner reflectors 95 is provided by the configuration of the shell S. Thus the reflectors units of the first two successive circumferential rows are on opposite sides of the point of maximum diameter of the shell and therefore are slightly canted relative to each other. The remaining successive circumferential rows are slightly oflset radially from each other by virtue of the taper of the target shell S.

The significance of the combination of relatively large radar corner reflector units with a plurality of relatively small radar corner reflector units may be appreciated by referring to FIGURES l1 and 12. FIGURE 11 shows the character of the reflection of radar signals from a corner reflector unit that is of eight times the dimension of the wavelength of the signal, the amplitude of the reflected signals being plotted for different directions of reflection as measured in degrees from one of the three reflecting surfaces of the unit with the transmitter located symmetrical to all three surfaces. The reflection attern in FIGURE 11 is what is commonly termed mono; static return since it comprises a central peak curve 96' that is of high amplitude but is relatively narrow, and includes only relatively narrow flanking peak curves 98 with numerous null points 100 at the junctures of the various peak curves. 4

FIGURE 12 shows the character of the reflection of radar signals from a corner reflector unit of a dimension of three and one third times the wave length of the radar signal. The reflection pattern is what is termed bi-static since it comprises a central peak curve 102 that is of substantially less amplitude but broader than the central peak in FIGURE 11, this peak curve 102 being flanked by only one or two lesser peak curves 104 that likewise are relatively broad. It is to be noted that there are only three null points 100 in FIGURE 12. The peak curves 104 at the opposite ends of the graph in FIGURE 12 are exceptionally broad because the radar reflections are greatly diffused at the relatively low angles, the diffusion being analogous to the diffraction of a light beam passing through a relatively small slot or orifice.

When it is considered that the described tow target construction provides such a large number of diversely oriented relatively small radar corner reflector units 95, it may be readily appreciated that numerous reflections from the relatively small corner units will'be received by a radar receiver at any given direction from the tow target. In this regard a feature of the preferredpractice of the invention, is that the tow target is caused to spin on its longitudinal axis, such spinning being readily permitted by the provision of the swivel member 52. To cause the desired spinning action, the four air foils of tail fins 30 are bent at their trailing ends, for example, along the lines 105 inFIGURE 1, to provide angular trailing portions 106 that function in the manner of an air screw to cause the desired spinning action.

The mono-static reflections from the relatively large corner reflector units of the forward and aftradial assemblies 70 and 72 make it possible for the described corner reflector'units 95.

identical reflecting surfaces. metrical corner reflector unit 95a which is produced-by thin shell.

flector'unit 95.

. 7 g 1 tow target to be picked up by radar detecting devices at relatively long distances, just as in the case of actual enemy aircraft. On the other hand, the numerous smaller bi-static radar corner reflector units 95 produce'somewhat weaker reflections but these reflections are so well dis- 7 accomplished by an arrangement that places the reflector units at random positions. small corner reflector units 95 are mounted on the inner faces of cellular plastic spacer blocks 198 and the spacer blocks'in turn are bonded tothe inner surface of the shell S to serveas spacers between the shell and the corner reflectors. The spacer blocks 168 are of random thicknesses andin addition may have inner faces of randominclination. In assemblying the structure illustrated by FIGURE 7, the spacer blocks 108 are "picked at ran-' dom' for installation to form a? circumferential series of the corner reflector units 95 and obviously no two series or circumferential rows of the reflector units will be alike. It is to be'noted that the random inclination of the inner faces of the spacer blocks 168 causes the reflector units to face in directions at random angles relative to the radii of the target body at therespective locations of the unit. V FIGURE 8 illustrates another procedure for insuring diversification in the orientation of the relatively'small;

In FIGURE 8, the circumferential row of the reflector units 95 is mounted on the inner surface of a tapered spacer band 110 of cellular plastic, the spacer band being bonded inturn to the inner surface of the shell S. In the arrangement shown, the minimum thickness points and the maximum thickness points of the spacer band 110 are positioned 180 apart. :It is' apparent that the successive corner reflector units 95 in the circumferential row are progressively offset radially from each other by a small distance.

FIGURE 9 illustrates how a further random factor may be introduced for diversification of the orientation of the plurality of relatively small radar corner reflector units. The corner reflector units 95 heretofore discussed In'FIGURE 7, the relatively V body, or may be carved into the body by suitable cutting' tools, or may be formed'by therapplication of -a'heated--' tool of the required shape, the heated tool melting the cellular 'plastic material to form the desired-peripheral recesses. The peripheral.recesses114- are then lined by insertionof the cardboard corner reflectorunits 95.-

Our description in specific detail of selected practices of the invention will suggestto those skilled in the art various changes, substitutions and other-departures from our disclosure that properly fall .withinfthe spiritand scope of the appended claims. a We claim: a

1. An aerial tow target, comprising: an elongated body of streamlined configuration; an axial member mounted in the nose portion ofsaid'body for attachment to a tow cable whereby said tow target may be towed by an aircraft to "which said tow cable is con,

nected; structural means internally of said body attached to said axial member for-transmitting towing stress to, said body; an electrical'component carried within .said body; propeller means 'rotatably surrounding said axial member, said propeller means including blade elements extending into the air stream. for deriving power therefrom as said tow target is towed by an aircraft; an -elec-. trical generator carried within said body; and means operv atively interconnecting said propeller means and :said elec-'. trical generator wherebyrsaid propeller. means rotatably drives said electrical generator in .the towed flight of the target for energizing saidelectrical component.

2'. An aerialtow target as set forth in claim 1 which includesair foils on the tow target body to cause the body to spin on its longitudinal axis; and which includes swivel.

means on the front endof said axial member ,for CD11:

" said electrical generator surrounds said axial member-i,

' 4. In a streamlined .radar reflectiverotary aerialtow;

are of conventional symmetrical design having three FIGURE 9 shows an asymfrelativeto the other two edges of the reflector unit.

* It isfcontemplated that the successive tapered corner -reflector units 95a in a series of such units will be turned to'various directions with respect to the location of the shortened corner edge 7111. Thus the successive tapered corner reflections will be selectively, effective in random directions. 4

FIGURE 10 illustratesanotherpractice of the invention, in whichthe body of the tow target is made of amass of cellular plastic material instead "of a relatively Thus FIGURE 10 shows a streamlined tow target body 112 of the. cellular .plastic'maiterialwhichbody may, if desired, 'be'hollow as shown, Numerous recesses 114 are formed in theperipher'y ofthe plastic body 1112 and each of the recesses conforms in configunection with a tow cable to permit the target-body .to spin independently of the cable.

3. Anaerial tow target as set, forth in claim2 in which target to simulate an enemy. aircraft for practice runs with radar target detecting devices employing .r-adar signals of a given wave length, means for reilecting said radar signals, comprising: -two"plura 1ities of outwardly facing radar corner reflectorspositioned-within thestreamlined configurationof said body, the dimension of; .one

of said pluralities of corner reflectors being more than V five times said wave length for substantially monostatic reflection of the radar signals for initial detection of the tow "target at relatively long distances, the dimension ofthe other of said pluralities ofxcorner reflectorsbeing less than five times said wavelength for-substantially T bistaticreflection of the radar signals whereby at'any in claimf' l in which said second plurality o corner re I 1 I ratiori to the shape of a're'latively sm'all'radar cornergre in claim" 7 areoriented inrandom' directions for, selectiveelfectl;

given moment in the rotation-of the tow target his highly V 1 E probable that at least oneof said reflectors of said other 1 of the pluralities will be positioned for diffused reflec- T 7 tion of the radarsignals to a detecting'receiyerin'a direction at a :low anglerelativeto one planar surface of said one corner-"reflector! V, v ;f

5. Means for reflecting the radar; signal flectors is offset inwardly from .theperipherymf:

body by various radialfdistances.

6. Meansfor reflecting torsface at various angles to ,radii' of the tow target for random r'eturn reflection of the radar: signals? second plurality is asymmetrical for selectiveefiecgtinone direction. p I

8'. Means; for reflecting the -rada.r signa ls; as-.se

in which said asymmetrical cornerlfrefle his in 'randorn directions:

9: Means r n 7 en ine are sass asset forth The recesses 114 may be molded into the in c1aim 4finwhich'the depth ofsaid monostaticcoriier 1 the radar signals asset forth 7 in claim l in which said'secondiplurality of cornergreflecreflectors is approximately the radial dimension of the aerial tow target at the respective locations of the reflectors.

10. Means for reflecting the radar signals as set forth in claim 4 in which the dimension of the monostatic corner reflectors is at least six and one-half times said wave length and the dimension of said bistatic corner reflectors is on the order of three to four times said Wave length.

11. In a streamlined radar reflective rotary aerial tow target to simulate an enemy aircraft for practice runs with radar target-detecting devices employing radar signals of a given wave length, means for reflecting said radar signals, comprising: a forwardly facing assembly of corner reflectors the dimension of which is greater than five times said wave length for substantially monostatic forward reflection of the radar signals; a rearwardly facing assembly of corner reflectors the dimension of which is greater than five times said wave length for substantially monostatic rearward reflection of the radar signals; and a plurality of cii'cumferenti'ally distributed corner reflectors located between said two assemblies, the corner reflectors of said plurality being less than five times said wave length for substantially bistatic reflec- References Cited in the file of this patent UNITED STATES PATENTS 1,860,982 Binnie May 31, 1932 2,146,723 Dunham Feb. 14, 1939 2,419,549 Griesinger Apr. 29, 1947 2,448,587 Green Sept. 7, 1948 2,463,517 Chromak Mar. 8, 1949 2,525,332 Alger Oct. 10, 1950 2,667,351 McKinney Jan. 26, 1954 2,795,778 Bagby June 11, 1957 2,805,065 Cotton Sept. 3, 1957 2,821,396 Seeley Jan. 28, 1958 

